1. Field of the Invention
The present invention relates to heat exchangers, and more particularly to heat exchangers featuring anti-freeze protection of the condensate draining path.
2. Brief Description of the Prior Art
Although freeze protection is an important criteria in designing an air-cooled steam condenser, the systems of the prior art, after more than two decades of development, still present complex and costly solutions to that problem and/or are unable to prevent freezing under certain operating conditions.
A typical solution to reduce the risk of freezing in the tubes of a steam condenser is to use a bundle of more than one row of tubes successively traversed by the air flow. The first row is struck by the coldest air flow but only a portion of the steam supplied to the tubes can be condensed. The outlet of the first row is connected to the inlet of the next row which converts a further amount of steam into condensate but is contacted by preheated air. Hence, although the steam could be totally reduced to cooled condensate at that stage, freezing is prevented because of the higher temperature of the air flow striking that row.
Larinoff in U.S. Pat. No. 5,787,970 issued on Aug. 4, 1998 presents an improved solution based on that concept characterized by a mixed flow vertical tube bundle design, in which some of the tube rows conduct counterflow steam and condensate while others have parallel flow. The condensate is drained at the bottom of the bundle from a header connecting a parallel flow row to a successive counterflow row in the protected warm air zone and non-condensable gases are collected at the outlet header of the counterflow rows.
The main drawback of the above type of systems lies in their lower efficiency/cost ratio as the second pass tube rows provide less heat exchange than the others for a comparable size and manufacturing cost. Also, some risks of freezing in the condensate drain piping and in tubes next to the edges of the bundle are still present. Moreover, circulation of steam and condensate in counterflow may result in interaction between the two fluids that disrupts normal flow and heat transfer. U.S. Pat. No. 5,056,592 (Larinoff) issued on Oct. 15, 1991 offers a solution to that problem by providing baffling inside some of the tubes to channel and separate the upward bulk flow of steam and the downward flow of condensate.
Another approach based on a similar principle is to use two rows of U-shaped tubes connected to a common steam supply as described in U.S. Pat. No. 3,705,621 (Schoonman) issued on Dec. 12, 1972. The tubes are so disposed that the air flow is successively striking the hottest legs of the first and second rows and then the coldest legs of the second and first tube rows.
Similarly, U.S. Pat. No. 4,926,931 (Larinoff) issued on May 22, 1990 presents a system in which the tubes are so arranged that steam flows from the input headers to the exposed legs of the inner and outer tube rows, and returns as condensate through the tube legs located in the protected warm air region in the middle of the tube bundle. The air flow thus successively strikes the hottest legs of the outer tube row, the coldest legs of the same row, the coldest legs of the second tube row and finally the hottest legs of that second row. Such an arrangement provides better protection to the exposed tubes especially at the top and bottom faces of the bundle. Moreover, the condensate drain headers extending in the protected region parallel and next to the steam supply headers provide some protection against freezing of the condensate by radiation heating. However, this system has drawbacks similar to the above concepts, as to the efficiency/cost ratio and still offers limited freezing protection especially in the U-shaped portions connecting the two legs of the finned tubes.
Another solution of comparable efficiency is described in U.S. Pat. No. 5,765,629 delivered to Goldsmith on Jun. 16, 1998 and uses a second stage vent condenser disposed in the same plane as a first stage condenser, both comprising bundles of vertically oriented tubes. The first stage operates at a higher steam pressure and consequently is easily drained from condensate and non-condensable gases into a lower header with excess steam. This header is connected to the upper header of the second stage condenser and to a hydraulically balanced common drain pot below the lower header. Non-condensable gases from the second stage flow counter-currently to be vented near the upper header. In this arrangement, freezing is controlled by continuous purging of the tube rows to avoid steam back-flow in the tube rows thereby eliminating trapping of condensate and non-condensable gases. However, this system is maintaining a constant level of condensate in the drain headers and the drain pot which are subject to freezing, particularly on the second stage condenser side.
Some solutions of the prior art have been specifically addressing potential freeze-up of the condensate drain lines. For instance, U.S. Pat. No. 3,968,836 (Larinoff) issued on Jul. 13, 1976 discloses a heat exchanger wherein leg seals connecting with outlets from individual condensate outlet headers are enclosed within a drain pot which is heated by uncondensed vapor from one of the outlet headers. In U.S. Pat. No. 4,240,502 issued on Dec. 23, 1980, Larinoff brings some improvements to the latter system, including a small hole in the drain pipe to purge the drain pot when the steam condensing system is shut down and applying some insulating material on the portion of the outlet header extending outside of the heated drain pot.
In U.S. Pat. No. 5,145,000, (Kluppel) issued on Sep. 8, 1992, a steam condensing system similar to the above has a tank receiving the condensate drain line from the drain pot. A steam line from the source of steam which also feeds the condenser, is connected to the upper end of the tank section receiving the drain line for supplying steam above the condensate level in the tank section. The steam heats the condensate drain line in the tank section to avoid freezing of the condensate.
In U.S. Pat. No. 5,355,943 (Gonano) issued on Oct. 18, 1994, steam from the source supplying the condenser is again connected to the upper end of a tank section receiving a condensate overflow drain duct from a drain vessel. Condensate is rain-like spread falling in the duct while the steam supplied to the tank goes up along the duct in countercurrent with the condensate, thus heating it on its passage to finally be sucked with non-condensable gases through the top portion of the drain vessel.
Although the latter vapor condensing system arrangements of the prior art significantly contribute to prevent freeze-up of the heat exchanger tube bundles or condensate drain lines, considerable drawbacks still limit their use on the market. Principally, their relative complexity significantly increases the system manufacturing and maintenance costs, while some efficiency of the heat transfer is lost and most of these systems still present risks of freezing especially if they are operated outside of their optimal vapour pressure conditions.
There is thus a need for an improved air-cooled vapor condensing system providing freeze protection over a wide range of operating conditions as required in applications such as heating of buildings.